Several years ago, (late 1980s), I laced up a Sachs 5-speed hub to a BMX rim.
3rd gear was straight through, so I had two gears lower than normal and two gears higher. That was quite a successful experiment, and resulted in a BMX with a pretty high top speed.

I've been thinking of doing it again with my current single-speed BMX, but this time use a Shimano 11-speed hub.
By my calculations, a cadence of 80 in top gear would propel the bike at about 40 kph (24 mph). It would be wild to overtake cyclists on $3000 carbon road bikes with a BMX, heh heh. Trouble is, those 11-speed hubs are very expensive.